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Tuning the Disorder of Superglasses

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Tuning the Disorder of Superglasses Tuning the disorder in superglasses Derek Larson1 and Ying-Jer Kao1,2, ∗ 1Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 106, Taiwan 2Center for Advanced Study in Theoretical Science, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 106, Taiwan (Dated: November 11, 2018) We study the interplay of superfluidity, glassy and magnetic orders in the XXZ model with random Ising inter- actions on a three dimensional cubic lattice. In the classical limit, this model reduces to a ±J Edwards-Anderson Ising model with concentration p of ferromagnetic bonds, which hosts a glassy-ferromagnetic transition at a crit- cl ical concentration pc ∼ 0.77. Our quantum Monte Carlo simulation results show that quantum fluctuations sta- bilize the coexistence of superfluidity and glassy order ( “superglass”), and shift the (super)glassy-ferromagnetic cl transition to pc >pc . In contrast, antiferromagnetic order coexists with superfluidity to form a supersolid, and the transition to the glassy phase occurs at a higher p. PACS numbers: 75.50.Lk, 75.40.Mg, 05.50.+q Introduction– Spin glasses are frustrated magnetic sys- Bose-Hubbard model with random frustrating interactions, in tems with quenched disorder, hosting glassy phases with ex- order to characterizetransitions by tuning the amountof disor- tremely slow dynamics[1]. Traditionally, the simplest model der present in the interactions. We demonstrate that the pres- that exhibits spin glass (SG) behavior is the ±J Ising model, ence of quantum exchange terms greatly increases the tem- or Edwards-Anderson (EA) model[2], in which Ising spins perature dependence of the SuG-FM transition by pushing the interact via randomly distributed nearest-neighbor (NN) fer- low temperature phase boundary to a higher critical concen- cl romagnetic (FM) and antiferromagnetic (AFM) bonds with tration than pc . The SuG-SS phase boundary is also drawn to probability p and 1 − p respectively. This model has been a higher critical concentration, and an asymmetry for SuG-SS studied extensively with Monte Carlo simulations and ex- and SuG-FM transitions arises. hibits a finite temperature glass transition in three dimensions Model– The Hamiltonian for hardcore boson with a ran- (3D) [3, 4]. It has been shown that this model has a second dom NN interaction is order transition from a SG to a FM (AFM) phase at a critical cl cl † † concentration pc = pc ∼ 0.77 (pc = 1 − pc ∼ 0.23) as H = − Vij (ni−1/2)(nj −1/2)−t (bi bj +bibj), (1) one increases (decreases) the fraction of FM bonds, and there hi,ji hi,ji exists reentrance of the spin glass phase in the temperature- X X disorder phase diagram[5–7]. One natural question to ask is where hi, ji indicates the NN lattice sites. ni is the number how the transition can be changedby quantumfluctuations [8– operator for hard-core bosons at lattice site i, t is the hopping 10]. In particular, understanding the behavior of granular su- parameter and Vij are interactions with a bimodal distribution perfluidity in a frozen amorphous structure may give us hints given as on the microscopic mechanism of the formation of supersolids (SS) [11]. p(Vij )= pδ(Vij − V )+(1 − p)δ(Vij + V ). (2) The observation of supersolidity in solid Helium 4 (He4) [11] has spurred immense interests on the connection between By a transformation of the bosonic operators to spin-1/2 oper- z − + † superfluidity and disorder in bosonic systems. Strong experi- ators: Si = ni − 1/2,Si = bi, and Si = bi , this model can mental evidence suggest that disorder may play a role in how be readily mapped into the standard XXZ model: the supersolid forms[12]. A recent experiment observed ultra- arXiv:1202.3908v3 [cond-mat.dis-nn] 28 Aug 2012 4 1 slow dynamics in solid He [13], suggesting a glassy type of H = − J SzSz − J (S−S+ + S+S−), (3) z i j 2 xy i j i j supersolid, or a “superglass” (SuG) [9, 10, 14, 15]. Quantum hi,ji hi,ji Monte Carlo (QMC) studies on the extended hardcore Bose- X X Hubbard model with random frustrating interactions on a 3D where Jxy = 2t and Jz = Vij . This model reduces to the cubic lattice indicate that glassiness can coexist with super- classical EA model if Jxy = 0. The FM and AFM phases fluidity, and quantum fluctuations and random frustration are in the spin model correspond to quantum solid phases with both crucial in stabilizing the superglass state[9]. (0, 0, 0) and (π,π,π) ordering vectors, respectively. In the While the phase transitions into superfluidity or quantum following, we will use the spin language for simplicity. solid/glassy states have been studied in some detail, the na- There exist both diagonal and off-diagonal long-range or- ture of the various transitions into a SS/SuG are yet left un- ders in this model. Possible diagonal long-range orders are explored. In this Letter, we study the XXZ model with ran- FM, AFM and SG, with corresponding order parameters: 1 z dom Ising interactions, which maps to an extended hardcore magnetization m = N i Si (FM), staggered magnetization P 2 1 i z 1.5 ms = N i(−1) Si (AFM), and the Edwards-Anderson or- der parameter (SG), defined as PM P 1 z 2 qEA = hSi i , (4) N 1 SG " i # AFM X av SuG FM where h· · · i denotes a thermal average and [··· ]av an aver- T age over disorder realizations. As the EA order parameter will also capture FM and AFM ordering, one must look for a 0.5 non-zero qEA while the other order parameters remain zero in order to identify an SG phase. To determine the phase transition point, we look at the SS Binder cumulants [16] for the order parameter, which should 0 cross at the transition point for different system sizes. How- 0 0.2 0.4 0.6 0.8 1 ever, the existence of corrections to scaling cause pairs of p small sizes to intersect away from the true critical point. To FIG. 1. (Color online) T vs p phase diagram. The dashed lines improve the accuracy of the measurement, we look at the se- indicate the position of the classical transitions SG-(A)FM, and the ries of intersection points created by successive pairs of sizes solid lines with arrows show the trend of the phase boundary shifts L and L +1 which should converge to the exact value as a we find in the quantum model. The blue circles are our estimates power law with the exponent determined by the leading cor- for the superfluid transition, with the approximate phase boundary rection to scaling. To get statistical estimates of these cross- drawn as the blue arc. Lastly, the red arc denotes the PM-SG phase ing points, we perform a bootstrap resampling on the raw data boundary. (Nboot = 100), fitting a 2nd-order polynomial to each size, and calculating the resulting intersection. The Binder cumu- TABLE I. Parameters of the simulations lants of the magnetization and staggered magnetization are β L p range ∼ Nsamp ∼ Nsweep 4 4 0.70 6 0.20 - 0.26 600 8000 1 hm i av 1 hmsi av gm = 3 − ; gsm = 3 − 0.70 8 0.20 - 0.26 400 16000 2 2 2 2 2 2 [hm i]av ! [hmsi]av ! 0.70 10 0.20 - 0.26 500 64000 (5) 0.70 12 0.20 - 0.26 250 128000 which approach one in the FM/AFM phase and goes to zero 1.00 3 0.70 - 0.80 2000 4000 otherwise. We measure the superfluid density through wind- 1.00 4 0.75 - 0.78 2000 16000 ing number fluctuations[17], 1.00 5 0.75 - 0.78 800 128000 1.00 6 0.75 - 0.78 150 256000 1.00 6 0.12 - 0.28 1000 16000 1 2 1.00 8 0.12 - 0.28 400 16000 ρs = hW i , (6) 3βNt k 1.00 10 0.12 - 0.28 400 64000 k=x,y,z X av 2.00 6 0.20 - 0.28 500 8000 where Wk is the winding number along the k direction. In 2.00 8 0.20 - 0.28 500 128000 our simulation, we identify the SS phase by the coexistence SSEa 3 0.75 - 0.79 4700 16000 of AFM and SF orders, and the SuG phase by the coexistence SSE 4 0.75 - 0.79 4000 16000 of SG and SF orders. SSE 5 0.75 - 0.79 4000 16000 Method– We use the Worm Algorithm QMC [18] to SSE 6 0.75 - 0.79 4000 32000 study this model, as well as the Stochastic Series Expan- a Various temperatures are simulated simultaneously in the parallel sion (SSE) [19] with the parallel tempering [20] near the tempering. FM boundary to access low temperatures[21]. We simulate many different realizations of the disorder–sets of interactions Vij –for a given parameter set. In the following, we choose phases: worm update loopstend to be long while in SF phases, V/t = 4 in order to maximize the extent of the superglass and under increasing ferromagnetic order the worms have a phase at p = 0.5[9]. Each realization is simulated indepen- low probability of hopping producing short loops. This ef- dently and equilibrium is determined by reaching measure- fect is significantly reduced in our SSE implementations since ments that stabilize within our error bars. Table I contains each MC step has an adaptively determined number of opera- the details of our simulation parameters. For our worm al- tor loop updates.
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